This implements recovery of a wallet from a Shamir's secret-sharing scheme.
Current limitations:
- we can only recover a wallet from an existing set of mnemonic shares, we don't generate new ones
- the mnemonic shares are not saved to storage (wallet file), so users can't see their "seed" via the "Wallet > Show" seed menu (the action is disabled in the GUI)
- we don't autodetect the wallet type, the user needs to click the option button and select "slip39" explicitely
This is a backport of [[https://github.com/spesmilo/electrum/pull/6917 | electrum#6917]] and [[https://github.com/spesmilo/electrum/pull/9059 | electrum#9059]]
Depends on D17576